Pseudoaxial

For 1,3-dithiolanes the ring is flexible and only small energy differences are observed between the diastereoisomeric 2,4-dialkyl derivatives. The 1,3-oxathiolane ring is less mobile and pseudoaxial 2- or 5-alkyl groups possess conformational energy differences (cf. 113 114) see also the discussion of conformational behavior in Section 4.01.4.3. 

Similar to cyclohexanones, substituted cyclopentanones also adopt a conformation with the substituents in a sterically favorable position. In the case of 2-substituted cyclopentanones 1 the substituent occupies a pseudoequatorial position and the diastereoselectivity of nucleophilic addition reactions to 1 is determined by the relative importance of the interactions leading to predominant fra s(equatorial) or cw(axial) attack of the nucleophile. When the nucleophile approaches from the cis side, steric interaction with the substituent at C-2 is encountered. On the other hand, according to Felkin, significant torsional strain between the pseudoaxial C-2—H bond and the incipient bond occurs if the nucleophile approaches the carbonyl group from the trans side. 

Propenyl)-1,3-dithiane, after lithiation and addition of zinc chloride, reacts with ethyl 2-oxopropanoate to give preferentially the. vvn-adduct37, which is an intermediate in the synthesis of racemic /ra .s-tetrahydro-2,3-dimethyl-5-oxo-2-furancarboxylic acid. It is assumed, that the ethoxycarbonyl group is brought to a pseudoaxial position in the cyclic transition state by the chelating zinc cation. 

Ternay and coworkers examined the conformations of thioxanthene-10-oxide (186) and related compounds. They found that oxygen preferred the pseudoequatorial position, 186e, but that the amount of pseudoaxial conformer, 186a, increased when the oxygen was complexed with iodine monochloride or trifluoroacetic acid171. 

If we apply the Mulak model of a pseudoaxial, D, symmetry (14), using a ratio c/a = 0.95, we find indeed a singlet ground state 0> and a first excited state 1> causing the observed TIP. Deviation from Di symmetry will affect to first order only the excited degenerate states and will not alter this simple... 

The "diamagnetism" or better the nonmagnetic ground state of Pu(C0T)2, was a specific test of different models for the electronic structure of the An(COT)2 compounds. It has been explained by a 0> ground state in a pseudoaxial De symmetry, interacting with the excited 1> state giving rise to the observed... 

When an additional methyl substituent is placed at C(3), there is a strong preference for alkylation anti to the 3-methyl group. This is attributed to the conformation of the enolate, which places the C(3) methyl in a pseudoaxial orientation because of allylic strain (see Part A, Section 2.2.1). The axial C(3) methyl then shields the lower face of the enolate.55... 

The anion of cyclohexanone /V,/V-dimclhyl hydrazone shows a strong preference for axial alkylation.122 2-Methylcyclohexanone N,N-dimethylhydrazonc is alkylated by methyl iodide to give d.v-2,6-dimclhylcyclohcxanone. The 2-methyl group in the hydrazone occupies a pseudoaxial orientation. Alkylation apparently occurs anti to the lithium cation, which is on the face opposite the 2-methyl substituent. 

C-Alkylations of l,4-dihydro-27/-pyrazino[2,l-A]quinazoline-3,6-diones at positions C-l and CM were studied in detail. Compounds of type 57 could be alkylated diastereoselectively at C-l, owing to the geometry of the piperazine ring, which is locked in a flat boat conformation with the R4 or R1 substituent in a pseudoaxial position to avoid steric interaction with the nearly coplanar C(6)-carbonyl group. Alkylation of 57 (R2 = Me, Bn, R4 = Me) in the presence of lithium hexamethyldisilazide (LHMDS) with benzyl and allyl halides resulted, under kinetic control, in the 1,4-trans-diastereomer 59 as the major product, with retention of the stereocenter at CM (Scheme 5). 

The above results justify a simple ligand field model for the bis(ligand) metal species which is based on the assumption of pseudoaxial symmetry (40). This model allowed a consistent reinterpretation of an early ESR study of Co(C5H5BPh)2 (13) (42) the reinterpretation was later confirmed by additional and more sophisticated ESR work on 13 (43,44). 

In this scheme, a highly puckered metallocycle was envisioned, possessing pseudoaxial and equatorial substituents, and reaction pathways were said to be favored which minimized the following effects (a) 1,3-diaxial interactions of substituents on the two a-carbons (b) axial substituent interactions with juxtaposed ring carbons and (c) 1,2-diequatorial interactions. This scheme predicts relatively nonstereospecific metathesis of rra/j.v-olefins but highly stereospecific metathesis of ra-olefins. For example, the following pathways for reactions of m-olefins were proposed ... 

The nAChR consists of five subunits arranged around a pseudoaxis of symmetry 198... 

The unsubstituted pyrrolo[l,2-f]oxazol-3-one 295 underwent a dihydroxylation with catalytic osmium tetroxide to exclusively give diol 296. This diol resulted from attack of the oxidizing agent from the concave face of the molecule due to the pseudoaxial protons that sterically hinder the /3-face to be attacked by the osmium reagent (Equation 51) <2004AJC669>. 

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